Nowadays, a growing number of petroleum wells are producing a mixture of oil and water. This phenomenon has been observed as in land as in sea installations.
The water coproduced with petroleum may flow free or be emulsioned in oil. The water-in-oil emulsion is present in wells, storage tanks, containers, pipe lines, transporting and in all the processing and refining steps.
Water and petroleum emulsions may be classified in three types:                1st. Water-in-oil (W/O) emulsions,        2nd. Oil-in-Water (0/W) emulsions,        3rd. Complex emulsions.        
A water-in-oil emulsion consists of water drops dispersed in a homogeneous petroleum phase whereas an oil-in-water emulsion is a dispersion of petroleum drops in an aqueous phase. This last one is known as an “inversed emulsion”. Complex emulsions are made of tiny drops of a phase suspended inside larger drops of another phase, which are suspended themselves in the first phase. Most common kind of emulsions in petroleum industry is W/O emulsions.
Considering water and crude oil as two defined phases, it must be established that the main criteria to define the type of formed emulsion is phase volume. It means that when both phase volumes are compared, the dispersed phase corresponds to that with a smallest volume and the dispersing phase to that with the largest volume.1,2 
The stability of water in crude oil emulsions depends strongly on the adsorption-desorption kinetics and rheological properties of the interfacial layer. This last one is formed as a consequence of the supramolecular interactions of the emulsifier molecules with high boiling points, as asphaltenes and resins, which decrease petroleum interfacial tension and provoke the water drop dispersion. The asphaltenes are molecules containing several condensed aromatic rings with different aliphatic and naphthenic substituents, which are able to pile up, coordinating simultaneously with water droplets and hindering their coalescence.
Emulsions may be classified in three types:    1. Weak emulsions: they break in few minutes and free water is immediately obtained.    2. Middle emulsions: they require at least 10 minutes to break up.    3. Squashed emulsions: A period of several hours or even days is needed to reach partial or complete separation.
Emulsions, being thermodynamically unstable, require certain energy to alter their equilibrium. Nowadays, physical and chemical methods are individually or sequentially applied to break an emulsion of water dispersed in crude oil. Concerning physical methods, the use of electrical fields and mechanical effect devices may be mentioned. These methods may be combined with heating, in order to increment the frequency and strength of collisions between dispersed water drops.1,2,3 
Chemical treatments to initiate a (W/O) emulsion breakage require the addition of products named demulsifiers, which are surfactants that weaken and break the interfacial layer, increasing by this way the water drop coalescence. The choice of the most convenient chemical demulsifier depends on various factors: its concentration, petroleum characteristics, emulsion mixing and optimal residence time.
Crude oil dehydrating implies mainly a problem of colloidal stability. It has been recognized that the colloidal stability is strongly influenced by the surface features of the drops dispersed in the continuous phase. There are two different types of colloidal stability: a first one which is a consequence of surface electrical charge, strongly dependent on the electrostatic repulsions, and a second one controlled by the adsorption of smaller particles on the drop surface (steric stabilization).
Studies of statistical mechanics and many-bodies physics have shown that the colloidal stability is a result of the energy-entropy balance of the system.4,5,6 The colloidal stability may be controlled modifying the drop surfaces, augmenting or decreasing the dispersed phase stability. The electrical charge of colloidal particles (in this case water drops) may be modified through pH, salt content and other physic-chemical parameters such as temperature, etc. It is apparent that the greater the surface charge, the greater the stability. Salt addition, of course, provokes a screening of the colloidal electrostatic repulsion and, in consequence, the aggregation of the colloidal particles is induced. These mechanisms are related to the energy control in the previous mentioned balance. On the other hand, the colloidal particle size and the presence of other kind of smaller neutral particles in the aqueous phase, contribute to entropy control. When two colloidal particles approach one to another, a polarization of the surface charge and a new arrangement of the adsorbed particles are given.4,7,8,9,10 
Demulsifiers used in petroleum industry are formulations of different families of chemical products (ethylene and propylene oxide copolymers, alkoxylated resins of alkylphenol formaldehyde, alkoxylated amines, alkoxylated epoxy resins, etc.) dissolved in one or several solvents such as toluene, xylene, short chain alcohols, naphtha, etc.)11 
Some important examples mentioned in literature and related to the use of demulsifiers as water-in-oil breakers in petroleum industry are the following:
The polyoxyethylene-polyoxypropylene-polyoxyethylene (POE-POP-POE) and polyoxypropylene-polyoxyethylene-polyoxypropylene (POP-POE-POP) copolymers produced by different companies receive names according to the initiator employed during their synthesis (in example propyleneglycol or ethylendiamine). Some structures are shown in (1).12,13

U.S. Pat. Nos. 2,425,845 and 3,334,038 protect the production process of copolymers with a structure (EO-PO-EO) pointing out the use of the following glycols as polymerization initiators ethyleneglycol, 1,2 propyleneglycol, 1,3-propyleneglycol, butyleneglycols, diethyleneglycols, dipropyleneglycols, triethyleneglycol, tripropyleneglycol and other additional aliphatic glycols.
U.S. Pat. No. 3,835,060 describes a process for emulsion breaking, using a formulation of polyglycol alkyl ether sulfates and polyoxyethylene-polyoxypropylene block copolymers. The chemical structure of the polyglycol alkyl ethers employed in this process is presented in (2), where R is the alkyl group (n=1-10 and M is an alkaline, alkaline earth metal or quaternary nitrogen (see formula 2, alkyl ether sulfated polyglycol). Emulsion breakage is reached after 120 minutes, when the mixture is dosed in a concentration interval between 20 and 140 ppm; although the type of crude oil is not specified, it is reported that a maximal separation of 35% of water is obtained along the demulsifier process.

U.S. Pat. No. 5,445,765 discloses demulsifiers made of polyethyleneimines alkoxylated with propylene and ethylene oxides, which may be employed successfully in a temperature interval between 10 and 130° C. and compositions from 0.1 to 200° C. These dehydrating agents were applied on a Western Africa crude oil, obtaining a separation of 47% in three hours. However, the crude oil composition is not mentioned in the patent.
U.S. Pat. No. 5,609,794 discloses the application of polyalkyleneglycol and ethylene oxide, which is esterified with an anhydride in order to form the diester. This compound is made to react with vinyl monomers and synthesized, by this way, different esters. The formulations are applied in a range of concentrations from 10 to 1500 ppm and in an interval between 7 to 80° C. The products are used to dehydrate petroleum (its characteristics are not mentioned) and other different refinery lines (turbosine, gasoline, lubricants, etc.). The document mentions that a separation of 40% of water is reached in few minutes.
U.S. Pat. No. 6,294,093 protects a demulsifying formulation consisting of dicarbamate compounds and a polyalkoxylated alkylphenol resin (see formula 3, alkylphenol resins); the formulations are constituted by water and organic soluble organic compounds, the formulations are introduced into the water-in-oil emulsion at concentration between 50 and 1000 ppm and the petroleum features are not mentioned.

U.S. Patent Publication No. 2004/0266973 describes an alkylphenol arylaldehyde alkoxylated polymer able to separate water-in-oil emulsions, including crude oil and other refined currents. It is applied as a formulation prepared in different organic solvents and naphtha, at concentrations between 1 and 3000 ppm, but the main characteristics of the treated crude oil are not mentioned.
WO 2007/115980 mentions that alkoxylated orto-esters can provoke the separation of water contained in an emulsion. The orto-ester general structure is shown in formula 4, where R1 is H or a hydrocarbonated chain, R2, R3 and R4 are groups C3-C4 alkylenoxy and/or ethylenoxy. The products described by this patent were evaluated in North Sea and Middle East crude oils and synthetic brine. A separation between 30 and 100% was determined for these products.

Polyoxyethylene-polyoxyproylene-polyoxyethylene (PEO-PPO-PEO) block copolymers have been functionalized with some amines as ammonia, ethylendiamine and polyethylendiamine, through the substitution of their corresponding α,ω-ditosyl esters (TsO-POE-POP-POE-OTs), this modification is followed by the condensation of the amino groups and the polyacrylic acid14.
It is mentioned in the cited international references that the described products are added into the crude oil emulsions as dissolutions or formulations including different types of commercial products, which act as coalescence or clarifier agents. It is also established that the different ethylene and propylene oxide units may be present in the polymer chain in any order.